We study the short-time relaxation dynamics of crosslinked and uncrosslinked networks of semi-flexible polymers using diffusing wave spectroscopy (DWS). The networks consist of concentrated solutions of actin filaments, crosslinked with increasing amounts of alpha-actinin. Actin filaments (F-actin) are long semi-flexible polymers with a contour length 1-100 mu m and a persistence length of 5-15 mu m; alpha-actinin is a small 200 kDa homodimer with two actin-binding sites. Using the large bandwidth of DWS, we measure the mean-square-displacement of 0.96 mu m diameter microspheres imbedded in the polymer network, from which we extract the frequency-dependent viscoelastic moduli via a generalized Langevin equation. DWS measurements yield, in a single measurement, viscoelastic moduli at frequencies up to 10(5) Hz, almost three decades higher in frequency than probed by conventional mechanical theology. Our measurements show that the magnitude of the small-frequency plateau modulus of F-actin is greatly enhanced in the presence of alpha-actinin, and that the frequency dependence of the viscoelastic moduli is much stronger at intermediate frequencies. However, the frequency-dependence of loss and storage moduli become similar for both crosslinked and uncrosslinked networks at large frequencies, G＇ (omega) proportional to G " (omega) proportional to omega(0.75+/-0.08). This high-frequency behavior is due to the small-amplitude, large-frequency lateral fluctuations of actin filaments between entanglements.